Structural panels of the sandwich type

ABSTRACT

A method of producing attachment zones or locations in a sandwich type panel made up of a cellular structure and of flat coverings disposed on both sides of the said structure and adhering to the latter, wherein, in spaced-out zones of this structure, which are approximately circular and have a diameter which is appreciably greater than that of a screw or other component and is, moreover, sufficiently great for each of the said zones to encompass a plurality of contiguous cells, a hardenable compound is poured into the cells in these zones in such a way as to fill them, while the said structure has still not received at least one of its two coverings, and then, after complete attachment of the said coverings and hardening of the compound, the centre of the nodes thus formed is pierced right through by a hole having substantially the shape and diameter necessary for the passage of a screw or other component.

United States Patent [191 Deplante [4 1 May 27, 1975 1 STRUCTURAL PANELS OF THE SANDWICH TYPE [75] Inventor: Henri Deplante, Paris, France [22] Filed: Mar. 30, 1973 [21] Appl. No.: 346,285

[30] Foreign Application Priority Data Mar. 31, 1972 France 72111660 [52] US. Cl. 156/253; 156/197; 156/291; 161/68; 52/617 [51] Int. Cl 331d 3/02 [58] Field of Search 161/68, 69; 29/455 LM; 156/197, 291, 295, 253; 29/526, 475, 471.1.

OTHER PUBLICATIONS Epoxy Resins, by Lee & Neville, 1957, pages 148 & 149.

Primary Examiner.lohn E. Murtagh Attorney, Agent, or FirmWilliam J. Daniel 1 1 ABSTRACT A method of producing attachment zones or locations in a sandwich type panel made up of a cellular structure and of flat coverings disposed on both sides of the said structure and adhering to the latter, wherein, in spaced-out zones of this structure, which are approximately circular and have a diameter which is appreciably greater than that of a screw or other component and is, moreover, sufficiently great for each of the said zones to encompass a plurality of contiguous cells, a hardenable compound is poured into the cells in these zones in such a way as to fill them. while the said structure has still not received at least one of its two coverings, and then, after complete attachment of the said coverings and hardening of the compound, the centre of the nodes thus formed is pierced right through by a hole having substantially the shape and diameter necessary for the passage of a screw or other component.

7 Claims, 6 Drawing Figures PAIENTEDW 2 7 m5 SHEET 1 STRUCTURAL PANELS OF THE SANDWICH TYPE BACKGROUND OF THE INVENTION The present invention relates to structural panels of the sandwich type comprising a cellular core of the honey-comb kind, on each side of which there adhere covering plates or sheets which are disposed perpendic' ularly to the partitions of the cellular structure.

The invention refers more particularly to the panels of this kind which are intended for the production of light, strong floors and partitions, especially in the construction of air transport vehicles. in these panels, as is already known, the cellular structure in the form of a honeycomb is generally made of light metal alloy, while the covering plates or sheets are made of plastics material which is attached to the cross-section of the partitions in the cellular structure.

For the attachment of panels of this kind to the framework intended to support them, use has hitherto been made of metal eyelets, which are often called inserts" and of which the two parts, which are engaged, on each side of the panel, in a hole previously formed in the latter with a tool, are riveted together by means of pressure, whereupon the plastics material is injected into the panel around the eyelet in order to fill up the space left in the cellular structure during the piercing of the hole intended for the location of the eyelet.

This method of procedure suffers from various draw backs. The eyelets are relatively expensive and greatly increase the cost price of the panel. They create a projection on each of the faces of the panel, so that the panels are supported only locally on the framework by the eyelets, thus contributing to bending between the latter under the effect of loads. Furthermore, the projection of the eyelets on the upper face of the panels causes wearing of the moquette with which the floor is generally covered. Finally, the resistance to tearing of eyelets located at the edge of the panels is not always adequate. Because of this the panels are ruined and have to be replaced.

SUMMARY OF THE lNVENTlON The present invention aims to make it possible to avoid these drawbacks and, in particular, to obtain a high degree of strength for the points of attachment of the panels, at minimal cost.

The object of the invention is to provide a method of producing attachment zones or locations in a sandwich type panel made up of a cellular structure and of flat coverings disposed on both sides of the said structure and adhering to the latter, wherein, in spaced-out zones of this structure, which are approximately circular and have a diameter which is appreciably greater than that of a screw or other component and is, moreover, sufficiently great for each of the said zones to encompass a plurality of contiguous cells, a hardenable compound is poured into the cells in these zones in such a way as to fill them, while the said structure has still not received at least one of its two coverings, and then, after complete attachment of the said coverings and hardening of the compound, he centre of the nodes thus formed is pierced right through by a hole having substantially the shape and diameter necessary for the passage of a screw or other component.

The hardenable compound is preferably based on plastics material.

The invention also extends to panels provided with the attachment nodes thus produced.

BRIEF DESCRlPTlON OF THE DRAWINGS The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view, in perspective, showing the make-up of a sandwich type panel;

FIG. 2 shows part of a finished panel;

FIG. 3 is a cross-section taken along the line llllll in FIG. 2; and

FIGS. 4, 5 and 6 are views in partial cross-section along the line lV-lV in H6. 2 and show successive phases in the production of an attachment node.

DESCRIPTION OF PREFERRED EMBODIMENTS ln order to produce a sandwich panel of the type under consideration, there is taken, as shown in FIG. 1, a cellular structure 1 of the honeycomb kind, which is most often made of light, aluminum-based alloy such as for example the alloy known under the designation A0 and of which the cells, which have a hexagonal or other shape, have a relatively small crosssection of the order of a few mm? In Fl(,. 1, only a few cells of the structure 1 have been illustrated, in perspective, the dimensions of their transverse section having been enlarged in order to provide clarity in the drawing. Flat coverings 3 and 4 are stuck to both sides of this structure, perpendicularly to the partitions 2 of the cells. In this way, the panel shown in FIGS. 2 and 3 is obtained.

In order to make up a floor or partition with panels of this kind the panels must be attached by their edges to a framework which is usually metal, and the problem consists in providing, along the edges of the panels, at a slight distance from these edges and at suitably dis tributed points 5 and also, if necessary, at other points on the panel, holes intended for the passage of screws or other components which will secure the panel to the framework.

As has been mentioned above, use has hitherto been made, at the points 5, of two-piece eyelets which are generally metal and are engaged, on each side of the panel, in holes previously formed in the latter with a tool, and which are riveted together by pressure, whereupon plastics material is poured all round the eyelet between the two covering plates of the panel in order to replace, between these plates, the cell walls which have been destroyed when the hole was pierced. For this purpose, the known eyelets comprise, on one of their collars, holes which permit the passage of an injection needle.

This method of procedure suffers from the drawbacks already mentioned.

According to the invention, a completely different procedure is adopted which avoids the use of metal eyelets.

One method of implementing the present invention involves taking the cellular structure 1, which is not yet provided with the coverings 3 and 4, and placing this structure on a slab 6. A plate or template 7 which is the same size as the panel to be produced is placed on the said structure and is pierced with apertures 8, the axes of which coincide with those of the holes 5 which are to be formed in the panel for the passage of attachment screws.

The apertures 8 in the template have a diameter which is very much greater than that of the holes with which the panel is finally to be provided. for example a diameter of about 20 mm. for a hole diameter of between 3 and mm. By means of a moving piston type pressure-injection gun 9. the nozzle of which has the same diameter as the aperture 8, there is injected. through each of these apertures. a compound made of plastics material which thus completely fills the cells revealed by the aperture 8, even if the cells remain partly masked at the periphery of this aperture.

This compound is such that it has good properties of adhesion to the coverings 3 and 4 which will subsequently be attached to the cellular structure.

The said compound is also such that it has thixotropic properties and thus remains in place in the cellular structure without escaping therefrom when the latter is removed from the slab 6 for the purpose of providing it with the coverings 3 and 4, preferably by hot pressing.

In the course of this operation. the compound which has been poured into the cells hardens and the finished panel thus contains. around the geometrical axes ofthe holes which are subsequently to be pierced, types of nodes in which the cells are filled with a hard plastics material between the coverings 3 and 4 (these zones have been shown in dotted lines at 11 in FIG. 2).

The panel is then taken and again placed on a slab 6a (FIG. 5) and has superimposed upon it the template 7, the apertures Sin which are thus again located opposite the nodes 11, whereupon. by means of a drilling gun 12 which is successively placed on each of the apertures 8, and of a diamond'tipped bit 13 which is fitted in the said gun. each hole 5 for the passage of an attachment screw is pierced in the centre of each of the nodes 11 which hole may possibly be milled at one or other of its ends.

The slab 6a preferably comprises a cut-out portion 6/) permitting the point of the bit 13 to pass through the panel. in the course of the operation. the assembly constituted by the panel and the template 7 is displaced on the slab [after the gun 12 has been raised vertically) in such a way as to bring each of the holes 8 in the template and each of the attachment nodes 11 under the said gun.

FIG. 6 shows. in section. one of the panel nodes pierced by the hole 5.

In the course of the operation involving the piercing of the hole 5, only those parts of the cell walls which are located in the cross-section of the hole 5 are destroyed. but all around the said hole, the walls of the cells which are filled with the hardened compound re main extant. which explains the remarkable strength of the attachment node thus produced.

Thus. an attachment location or node is obtained. without the addition of metal components. which exhibits remarkable properties in respect of strength while still remaining economical to produce and avoid ing any detrimental projection on the faces of the panel. which remain perfectly fiat.

As an example of a compound which is suitable for pouring into the cells in order to make up the nodes. attention may be drawn to a mixture of a fluid resin containing its hardening catalyst. and of a very high proportion of finely ground silica (about 250 parts by weight of silica to l()() parts by weight of resin). The ground silica tranforms the fluid resin into a thixotropic product and gives it, after hardening, exceptional hardness and tensile strength. The resin will advantageously be an epoxy resin in the event of the coverings 3 and 4 themselves containing a resin of this type.

It should be noted that an epoxy resin-based compound of this kind is capable of hardening at ambient temperature. which justifies its use for repairs. After polymerisation, the product is self-extinguishing.

The method of implementation described could naturally be modified in various ways.

Thus. for example. in the phase in FIG. 4, the use of a special thixotropic compound could be avoided if a start were made by sticking one of the covering plates, for example the plate 4, to one of the faces of the cellu lar structure in such a way that this plate would serve as a base for the pouring of the plastics material into the cells of the honeycomb.

When the panel is finished. it is advantageous to reinforce its edges by milling the cellular structure over a distance of a few mm. between the plates 3 and 4 and by then filling the part thus evacuated, which is shown in shaded form on the left of FIG. 3, with a suitable hardenable thixotropic compound. for example. with a compound similar to the one mentioned above.

The method according to the present invention is not restricted to the above described example but is capable of modification without departing from the scope thereof as defined by the appended claims.

What is claimed is:

1. In a method of producing sandwich panels by applying flat coverings, each including plastic material, on both sides of a cellular honeycomb-like structure, the improvement of providing reinforced nodes at selected spaced apart points of the panel for the attachment thereof to a supporting structure. by the steps comprising: while said cellular structure has at least one of its sides uncovered, completely filling each of a plurality of adjoining cells of said structure surrounding each of said points in a zone having the node point as the center thereof with a hardenable fluid resin having good adhesion to the plastic material of the coverings by pouring said resin in liquid state into each of said cells; applying the remaining covering to the cellular structure while plural cells at each such point are filled with said fluid resin; hardening said resin after both said coverings are in place; and thereafter, piercing through the coverings and the filled cellular structure along the axis of each node.

2. A method according to claim 1 wherein said hardenablc fluid resin is a resin epoxy containing a hardening catalyst therefor.

3. A method as claimed in claim 1, in which said hardenable fluid resin is thixotropic so that, even before hardening. it remains in the cells filled therewith.

4. A method as claimed in claim 3. in which the hardenable resin comprises a mixture of fluid resin and finely ground silica.

5. A method as claimed in claim 1, in which the resin is poured into the cells of the cellular structure before either of said coverings has been secured to the structure. and while said structure is supported on a fiat surface.

6. A method as claimed in claim 1. in which a template having apertures is superimposed upon the said structure for the pouring of the resin into the spacedapart zones of the cellular structure. the axes of said apertures coinciding with those of the holes to be finally 6 formed in the panel. but having a diameter which is apsaid structure, the holes are formed in said nodes by preciably greater than that of the said holes. means of a drilling gun which is placed on the apertures 7. A method as claimed in claim 6, in which after the in the said template. resin has been hardened in the spaced-apart zones of 

1. IN A METHOD OF PRODUCING SANDWICH PANELS BY APPLYING FLAT COVERINGS, EACH INCLUDING PLATE MATERIAL, ON BOTH SIDES OF A CELLULAR HONEYCOMB-LIKE STRUCTURE, THE IMPROVEMENT OF PROVIDING REINFORCED NODES AT SELECTED SPACED APART POINTS OF THE PANEL FOR THE ATTACHMENT THEREOF TO A SUPPORTING STRUCTURE, BY THE STEPS COMPRISING: WHILE SAID CELLULAR STRUCTURE HAS AT LEAST ONE OF ITS SIDES UNCOVERED, COMPLETELY FILING EACH OF A PLURALITY OF ADJOINING CELLS OF SAID STRUCTURE SURROUNDING EACH OF SAID POINTS IN A ZONE HAVING THE NODE POINT AS THE CENTER THEREOF WITH A HARDENABLE FLUID RESIN HAVING GOOD ADHESION TO THE PLASTIC MATERIAL OF THE COVERINGS BY POURING SAID RESIN IN LIQUID STATE INTO EACH OF SAID CELLS, APPLYING THE REMAINING COVERING TO THE CELLULAR STRUCTURE WHILE PLURAL CELLS AT EACH SUCH POINT ARE FILLED EITH SAID FLUID RESIN, HARDENING SAID RESIN AFTER BOTH SAID COVERINGS ARE IN PLACE, AND THEREAFTER, PIERCING THROUGH THE COVERINGS AND THE FILLED CELLULAR STRUCTURE ALONG THE AXIS OF EACH NODE.
 2. A method according to claim 1 wherein said hardenable fluid resin is a resin epoxy containing a hardening catalyst therefor.
 3. A method as claimed in claim 1, in which said hardenable fluid resin is thixotropic so that, even before hardening, it remains in the cells filled therewith.
 4. A method as claimed in claim 3, in which the hardenable resin comprises a mixture of fluid resin and finely ground silica.
 5. A method as claimed in claim 1, in which the resin is poured into the cells of the cellular structure before either of said coverings has been secured to the structure, and while said structure is supported on a flat surface.
 6. A method as claimed in claim 1, in which a template having apertures is superimposed upon the said structure for the pouring of the resin into the spaced-apart zones of the cellular structure, the axes of said apertures coinciding with those of the holes to be finally formed in the panel, but having a diameter which is appreciably greater than that of the said holes.
 7. A method as claimed in claim 6, in which after the resin has been hardened in the spaced-apart zones of said structure, the holes are formed in said nodes by means of a drilling gun which is placed on the apertures in the said template. 